Stereopair
of the Cloisters, New College,
Oxford. The right photograph was taken,
the camera was shifted about 3 inches to the left, and the left photograph was taken.

7 THE
CORPUS CALLOSUM AND
STEREOPSIS

The corpus callosum, a huge band of myelinated fibers, connects the two
cerebral hemispheres. Stereopsis is one mechanism for seeing depth and judging distance. Although these two features of the brain and vision are not
closely related, a small minority of corpus-callosum fibers do play a small role
in Stereopsis. The reason for including the two subjects in one chapter is
convenience: what I will have to say in both cases relies heavily on the special
crossing and lack-of-crossing of optic nerve fibers that occurs at the optic chiasm
(see illustration on p. 14), and it is easiest to think
about both subjects with those anatomical peculiarities in mind.

THE
CORPUS CALLOSUM
The corpus callosum (Latin for "tough body") is by far the largest bundle of nerve fibers in the entire nervous system. Its population has
been estimated at 200 million axons—the true number is probably higher,
as this estimate was based on light microscopy rather than on electron microscopy—
a number to be contrasted to 1.5 million for each optic nerve and 32,000
for the auditory nerve. Its cross-sectional area is about 700 square millimeters,
compared with a few square millimeters for the optic nerve. It joins the
two cerebral hemispheres, along with a relatively tiny fascicle of fibers called
the anterior commissure, as shown in the two illustrations on the following pages.
The word commissure signifies a set of fibers connecting two homologous neural structures on opposite sides of the brain or spinal cord; thus the corpus
callosum is sometimes called the great cerebral commissure.
Until about 1950 the function of the corpus callosum was a complete mystery. On rare occasions, the corpus callosum in humans is absent at birth,
in a condition called agenesis of the corpus callosum. Occasionally it may
be completely or partially cut by the neurosurgeon, either to treat epilepsy
(thus preventing epileptic discharges that begin in one hemisphere from spreading
to the other) or to make it possible to reach a very deep tumor, such as
one in the pituitary gland, from above. In none of these cases had neurologists and
psychiatrists found any deficiency; someone had even suggested (perhaps not
seriously) that the sole function of the corpus callosum was to hold the two
cerebral hemispheres together. Until the 1950s we knew little about the detailed connections of the corpus callosum. It clearly connected the two cerebral hemispheres, and on the basis of rather crude neurophysiology it was thought to join precisely corresponding cortical areas on the two sides. Even
cells in the striate cortex were assumed to send axons into the corpus callosum to
terminate in the exactly corresponding part of the striate cortex on the opposite
side.
In 1955 Ronald Myers, a graduate student studying under psychologist Roger Sperry at the University of Chicago, did the first experiment that
revealed a function for this immense bundle of fibers. Myers trained cats
in a box containing two side-by-side screens onto which he could project images,
for example a circle onto one screen and a square onto the other. He taught
a cat to press its nose against the screen with the circle, in preference to the
one with the square, by rewarding correct responses with food and punishing mistakes mildly by sounding an unpleasantly loud buzzer and pulling the cat back
from the screen gently but firmly. By this method the cat could be brought
to a fairly consistent performance in a few thousand trials. (Cats learn slowly;
a pigeon will learn a similar task in tens to hundreds of trials, and we
humans can learn simply by being told. This seems a bit odd, given that a cat's
brain is many times the size of a pigeon's. So much for the sizes of brains.)
Not surprisingly, Myers' cats could master such a task just as fast if
one eye was closed by a mask. Again not surprisingly, if a task such as choosing
a triangle or a square was learned with the left eye alone and then tested
with the right eye alone, performance was just as good. This seems not particularly impressive, since we too can easily do such a task. The reason it is easy
must be related to the anatomy. Each hemisphere receives input from both eyes,
and as we saw in Chapter 4, a large proportion of cells in area 17 receive input
from both eyes. Myers now made things more interesting by surgically cutting
the optic chiasm in half, by a fore-and-aft cut in the midline, thus severing
the crossing fibers but leaving the uncrossed ones intact—a procedure
that takes some surgical skill. Thus the left eye was attached only to the left hemisphere and the right eye to the right hemisphere. The idea now was to teach the
cat through the left eye and test it with the right eye: if it performed correctly,
the information necessarily would have crossed from the left hemisphere to
the right through the only route known, the corpus callosum. Myers did the
experiment: he cut the chiasm longitudinally, trained the cat through one
eye,
and tested it through the other—and the cat still succeeded. Finally,
he repeated the experiment in an animal whose chiasm and corpus callosum had both been surgically divided. The cat now failed. Thus he established,
at long last, that the callosum actually could do something—although we
would hardly suppose that its sole purpose was to allow the few people or animals with divided optic chiasms to perform with one eye after learning a task
with the other.

The
corpus callosum is a thick, bent plate of axons near the center of this brain section, made by cutting apart the human cerebral hemispheres and looking at the cut surface.

Here
the brain is seen from above. On the right side an inch or so of the top has been lopped off. We can see the band of the corpus callosum fanning out after crossing,
and joining every part of the two hemispheres. (The front of the brain is at the top of the picture.)